1. Field of the Invention
This invention relates to an apparatus and method for drying or curing inks, and more particularly, to an apparatus and method for utilizing electrical resistance heating and directed airflow to dry or cure screen printed ink on a material such as a textile.
2. Description of Related Art
Conventional silk screen presses print multi-colored images on material by mounting the material to a platen and rotating the platen past each of a plurality of print units, located peripherally about a central support, wherein each print unit prints a different color. The images printed at the several print units, when superimposed one over the other on the material, produce the desired multi-colored work. It is important in multi-colored apparatus of this type to completely cure, or dry, the ink applied at a previous station prior to application of a differently colored ink at a subsequent station. Curing between successive ink applications is sometimes necessary or desirable in order to avoid smearing or blurring of the previously printed image upon printing of a subsequent image thereupon. It is known to utilize electrical resistance heating elements situated in proximity with the printed material, or workpiece, which impart radiative and convective heat to the printed material between successive printing operations sufficient to cure the print thereon. Since only a thin layer of ink is applied in silk screening applications, the required heat exposure time for curing is relatively short. Overheating of the printed work may result in wrinkles, discoloration, shrinkage, and/or scorching of both the applied ink and the underlying material. Therefore, it is important that the heat application be closely controlled.
Normally, only a thin layer of ink is applied in screen printing operations, and exposure to ambient air is sufficient to adequately cure or dry the applied ink. However, certain applications require a heavier layer of applied ink. For instance, before printing fluorescent ink upon a black material such as a T-shirt, it is necessary to apply a heavy layer of white ink to completely cover up the black substrate. Thus, curing is generally necessary immediately following such a white layer print unit at which the heavy layer of white ink is applied.
Ink may be cured at any number of different stations between different printing operations for various reasons. Therefore, it is desirable to provide a curing apparatus which is portable so that the apparatus can be inserted and removed at any desired printing station or open station about the screen press as desired to suit a given application. Thus, one or two portable curing apparatus may be located about the press as desired to suit a given application to perform the desired curing operations, instead of maintaining a multiplicity of stationary curing apparatus about the press and only employing one or two of these in any given application. Significant savings are realized by the ability to reduce the number of curing apparatus required. Therefore, there is a need for a portable curing apparatus for use with silk screen printing presses.
The heat generated by electrical resistance heaters increases with time upon application of a given voltage, so that the heating elements require time to reach their point of maximum heat emission. A particular problem with current designs is that they are not able to reconcile the conflicting goals of providing maximum heating during curing and interruption of heat to the printed material between curing operations, without significantly reducing production speed. Two alternative methods are employed in current curing apparatus. Either the heating elements are maintained at a constant high heating level so that no time is lost in bringing the heating elements back up to their maximum level, or else the voltage supplied to the heating elements is completely interrupted between curing operations and reapplied during curing. Both designs have been found to be inadequate. Maintaining the heating elements at high voltage has been found to cause high heat build-up, and if the apparatus is stopped from indexing the workpiece, a paper or textile workpiece can be subjected to sufficient heat to scorch or to catch fire. To prevent scorching or burning of the material when the indexing movement is stopped, the voltage may be interrupted. However, interruption of the voltage to the heating elements between curing operations is undesirable in that production speeds are limited by the time required for the heating elements to reattain their maximum heating level upon reapplication of electrical power thereto.
An example of apparatus wherein the heating elements are maintained at a constant heating level is the current shutter concept. This design employs a conventional shutter system positioned between the printed material and the heating elements to isolate the heating elements from the printed material at the completion of the desired curing time. Therein, a plurality of slats pivot simultaneously between an open position, wherein the slats are parallel one another so as to allow a generally free flow of heat and air therethrough, and a closed position, wherein the slats overlap one another to block the flow of heat therethrough. Only during production interruptions is the flow of electrical power to the heating elements interrupted.
As with other portable curing apparatus employing heating elements maintained at the high curing temperature, this design has been found to be undesirable in several respects. Since the function of the shutters is to block off the open face of the housing between curing operations to prevent heat from exiting therefrom, an undesirable accumulation of heat within the housing develops. This results in the overall apparatus, including the shutters, becoming hot. Thus, during production interruptions, wherein power to the heating elements is interrupted, the heated apparatus continues to impart undesirable heating to a printed material situated therebeneath. There is a need to prevent this residual heating.
Also, the aforementioned accumulation of heat, in combination with the high temperature produced by the heating elements, has been found to cause warping of the shutters. Such warping results in the inability of the shutters to effect an adequate seal between the heating elements and the workpiece, thereby allowing radiative and convective heat to be imparted to the printed material between curing operations. Warping of the shutters has also been found to cause malfunctions in the opening and closing thereof. Furthermore, the high temperature of the heating elements and heat accumulation has been found to cause excessive metal fatigue in nearly all movable components of the apparatus, which further contributes to an undue number of shutter malfunctions. Accordingly, it is desirable to provide a portable curing apparatus in which the heating elements are not maintained at a continual high temperature.
Likewise, curing apparatus in which the electrical supply to the heating elements is completely interrupted between curing operations have been found undesirable as well. While the heat accumulation problems associated with constant heating of the heating elements is minimized, the problem with such designs is that their production capacity is relatively slow. That is, since the heating elements require time to reattain the desired heating level for curing after each power interruption, the production rate is limited by this lag time. Since advances in press speeds are constantly being realized, it is desirable to provide a curing apparatus which does not require significant time to reheat the heating elements between each curing operation. For example, T-shirt screen printers may index the pallet holding the T-shirt every four to six seconds. Stopping of the indexes for 20 or 30 seconds to reattain the desired heating level will slow production.
Of additional importance in such apparatus is the ability to completely interrupt the application of heat to the printed material after the desired curing period. Current curing apparatus, and most particularly portable curing apparatus due to their minimized machinery, suffer in their inability to prevent residual heat from the apparatus from continuing to heat the printed material even after complete discontinuation of electrical flow to the heating elements. That is, the electrical resistance heating elements become heated to very high temperatures and require time to cool down, even after interruption of electrical power. Also, the electrical resistance heating elements employed to impart the desired heat for curing, heat not only the printed material, but also heat the apparatus supporting and housing the heating elements. After the desired curing time has elapsed, and the flow of electricity to the heating elements is interrupted, it is important to prevent the heated curing apparatus and heating elements from continuing to impart heat to the workpiece. Otherwise, accurate curing times are not attainable and overheating may result.
While it is possible to compensate, at least in part, for such residual heating wherein the apparatus is in continuous operation with printed materials regularly advanced, such residual heating is particularly problematical wherein it is necessary to interrupt production for any reason, thereby leaving printed material stationary beneath the curing apparatus. Generally, the length of such interruptions is unknown, and during such interruptions a printed material may be situated in proximity with the heated curing apparatus which will continue to impart convective heat to the printed material throughout the duration of the interruption. This uncontrolled residual heating precludes the aforementioned requisite heating accuracy.
Therefore, there is a need for a curing apparatus capable of imparting an accurately controlled amount of heat to a printed material, and thereafter preventing any residual heat of the apparatus from effecting the printed material. It is desirable to provide such an apparatus which does not employ mechanical means in proximity with the heating elements so that fatigue effects are minimized. Also, since curing apparatus are not necessary for many applications, it is desirable to provide such an apparatus which is portable, so that the apparatus can be relocated to any position about the printing press, or removed completely from the press, as required to suit a desired operation. It is also desirable to provide such apparatus wherein the height of the heating elements in relation to the printed material can be accurately and easily adjusted so as to provide flexibility to allow the apparatus to be utilized in conjunction with a wide variety of printing presses, and also allow for more accurate control of the amount of heat from the heating elements imparted to the printed material. Furthermore, it is desirable to provide such a curing apparatus which lends itself to economical manufacturing and operation.